In continuous steel casting, in order to discharge molten steel from a tundish into a mold, an immersion nozzle is used. The immersion nozzle is used while being joined to an upper refractory such as an upper nozzle, a sliding nozzle plate, or a lower nozzle, wherein among others the immersion nozzle is worn out by the molten steel and so forth, so that the method is known with which only the immersion nozzle is replaced during continuous casting.
In this replacement method, a used (old) immersion nozzle is replaced by pushing it out with a new immersion nozzle, so that the replacement can be done under the state that the immersion nozzle is immersed in a mold during continuous casting. With regard to the method for replacing the immersion nozzle during continuous casting, in order to minimize a leakage of the molten steel during replacement, the method is disclosed, for example, in Patent Document 1, wherein the replacement is carried out by sliding both the new and used immersion nozzles while being pressed upward to the upper refractory such as the upper nozzle, the sliding nozzle plate, or the lower nozzle.
In the replacement method of Patent Document 1, as depicted in FIGS. 10a-10c the flange portion 53 of the used immersion nozzle 52 (or still in use) is biased upward with the keyboard row 51 arranged in both sides thereof so as to be kept under the state of being pressed to the joint interface 54 of the upper nozzle 56; therefore, when the immersion nozzle 52 is replaced, the new immersion nozzle 52a is pushed toward a lateral direction with the pusher 58 that is connected to the cylinder 57 so as to replace the used immersion nozzle 52. At this time, the new immersion nozzle 52a is caused to slide while being pressed to the joint interface 54 of the upper nozzle 56, so that the immersion nozzle can be instantly replaced without causing leakage of the molten steel even during continuous casting.
However, in this replacement method, the upper nozzle and the immersion nozzle are pressure-joined between the refractory joint planes; therefore, a space can be formed occasionally between the joint planes due to the local abrasion during replacement work as well as the thermal expansion during use thereof or the variance of the plane accuracy at the time of production thereof. If the space is formed, there are risks of quality deterioration of the steel due to suction of an air through this space, and of leakage of the molten steel from the space.
On the other hand, in the case that the replacement method like this is not carried out, in general the immersion nozzle and the upper nozzle are joined via a shaped joint sealer so as to ensure the sufficient sealability. The shaped joint sealer is a refractory in the form of a flexible sheet having a cutout portion with the size as same as or a slightly larger than a nozzle hole of the immersion nozzle to be used, wherein this sealer is deformed upon pressing the immersion nozzle to the upper nozzle so that it can fill the space (Patent Documents 2 to 6). Some of the shaped joint sealer have flexibility in a wide temperature range from normal temperature to hot.
However, in the replacement method of Patent Document 1, the new immersion nozzle was caused to slide under the state that it was pressed to the upper nozzle; and thus, even the shaped joint sealer was arranged on the upper plane of the new immersion nozzle, this shaped joint sealer was scraped off or taken out by the upper nozzle, so that the shaped joint sealer could not be used.
Hence, the method for replacing the immersion nozzle in which the shaped joint sealer can be used is disclosed in Patent Document 7. In the replacement method of Patent Document 7, the new immersion nozzle is moved to below the upper nozzle with keeping a certain space with the upper nozzle's lower plane, so that the shaped joint sealer arranged on the upper plane of the new immersion nozzle can be kept in the state of being originally arranged on the immersion nozzle's upper plane without contacting to the upper nozzle during the immersion nozzle is moving.
However, with the replacement method of Patent Document 7, a space is formed between the new immersion nozzle and the upper nozzle during replacement, so that there is a problem that the molten steel drops on the upper plane of the new immersion nozzle thereby becoming foreign matters of the joint interface, resulting in decrease of the sealability. Meanwhile, during replacement, the flow of the molten steel is stopped by a stopper or the like, but the molten steel remaining in the nozzle hole drops.